Various problems associated with the use of extremely sensitive sensors are well known. One principal problem encountered is that of dynamic range. A very high sensitivity detector will, of necessity, be required both to accommodate extremely small detected signals and at the same time avoid saturation in the presence of large amplitude interference signals. This introduces a conflict with the need for high gain in order to amplify small signals and lift them above the noise floor of subsequent stages, such as in analogue to digital (A/D) converters, since such gain cannot be so large that it leads to saturation from large interference signals.
The principal interference signals are usually caused by mains related frequencies. These are normally well defined in frequency, but may vary both in amplitude and in spectral composition of the various harmonics of 50 Hz (or 60 Hz). This situation has been exacerbated by the now widespread use of switch mode power supplies in almost all electronic appliances. In many cases these power supplies create more high order harmonics, of significantly larger amplitude, than their equivalent linear circuits. There are a number of approaches which may be used to alleviate the problem of large 50 Hz (or 60 Hz) and harmonically related interference signals.
In the case of magnetometers, a common solution is to use two or more sensors arranged in a gradiometric configuration to attenuate the effect of distant sources. This, however, produces a system which is sensitive to field gradients instead of fields, thus rendering it unsuitable for applications where the source is remote from the sensor.
Another solution, in the case of magnetometers, is to filter out the frequency components corresponding to the interference signals, either by means of analogue filters or by employing digital filtering techniques after the signal has been digitised. A problem with the latter approach, however, is that digitising the signal may produce a large dynamic range between the wanted signal and the interference signal. The ratio may easily reach 1:3×107 or 150 dB in an open unshielded environment. As a result, the subsequent stage, for example an A/D converter, would require at least 24 bit accuracy and a sub-microvolt noise floor. A much simpler and more robust solution is to implement analogue filtration in hardware. This attenuates the amplitude of the interfering signals considerably and thereby reduces the requirement for such a large dynamic range and low noise floor in the subsequent stage or A/D converter. For example, an attenuation of 60 dB in an interference signal would reduce the dynamic range requirement to 90 dB, a level which may easily be handled by a 16 bit A/D system.
As an illustration of the magnitude of the problem, a top of the range 24 bit A/D converter is stated as having only 110 dB of dynamic range with 45 kHz of bandwidth. This A/D converter is seven times more expensive than a typical 16 bit A/D converter which will display >90 dB of dynamic range and >100 kHz of bandwidth. Clearly, the 24 bit converter is not only a very expensive option, but it also fails to meet the above requirement for a 150 dB dynamic range.
Although the problems relating to the use of extremely sensitive sensors are described above in relation to magnetometers, nevertheless the competing requirements of low noise and high dynamic range are common to many sensor systems, including electrical potential sensors as well.
It is worth noting that in almost all real world applications of sensors, the sensor sensitivity is limited not by the inherent capability of the sensor or the instrumental noise but by the ability of the system to encompass the full dynamic range of environmental noise and interference.
There is, therefore, a very significant need for a highly sensitive sensor, in which the problems of interference are satisfactorily addressed.